Method for preparing cold cathodes and resulting article



B. G. FIRTH 0,994 METHOD FOR PREPARING cow CATHODES AND RESULTINGARTICLE;

Aug. 30, 1960 Filed Feb. 25. 1959'- INVENTOR JEWP/VARD 4i 5P7 film LATTO R N EYS METHOD FOR PREPARlNG COLD CATHODES AND RESULTING ARTICLEBernard G. Firth, Newark, NJ., assignor to Tang-Sol Electric Inc., acorporation of Delaware Filed Feb. 25, 1959, Ser. No. 795,514

9 Claims. (Cl. 117222) The present invention relates to electricdischarge devices of the type wherein the current carriers are electronsemitted from a coated electrode and comprises a coating for such typeelectrode which has improved electron emissive characteristics. Theinvention includes also a method of preparing and applying a coatingcomposition to the metal of the electrode which is commerciallypracticable and insures optimum physical condition of the appliedcoating. The invention will be described with reference to a cathodecoating for a cold cathode vacuum tube of the type disclosed in thecopending joint application of the present applicant and Albert M.Skellet, Serial No. 697,928, filed November 21, 1957, and entitledStarting Means for Cold Electrode Vacuum Tube Devices.

It has heretofore been known that a magnesium oxide coating, when soprepared as to be porous and formed of microscopic needle-like orstalactite-like structures, will give off a copious and self-sustainingstream of electrons when bombarded with primary electrons. (SeeDobischek et al. Patent No. 2,802,127.) No commercially practical methodof forming such coating has heretofore been known nor have cold cathodevacuum tubes been produced heretofore which could carry relatively highcurrents. For example, about 30 milliamperes per square centimeter ofcoating is about the maximum current obtainable from magnesium oxidecoatings of the prior art. In accordance with the present invention coldcathodes have been prepared that are capable of sustaining currents ofthe order of 145 milliamperes per square centimeter.

In accordance with the invention a mixture of mag nesium oxide, basicmagnesium carbonate powder and a thinner is first prepared. The mixtureis then milled in a ball mill jar with pebbles of selected graded sizefor an extended period of time and at a predetermined rate of rotation.After milling additional thinner is added and the so prepared coating issprayed on the electrode while the electrode is rotated at apredetermined relatively high rate. Each of the above described steps ofthe new process contributes to the improvement in the quantity ofelectron emission from the coated electrode.

The single figure of the accompanying drawing is a perspective view,partly broken away, of a cold cathode tube having a coated cathodetherein prepared in accordance with the invention.

The cold cathode vacuum tube illustrated in the drawing comprises anouter glass envelope 2 sealed to a button type glass base 4. Acylindrical cathode 6 prepared in accordance with the invention ismounted on the base 4 by means of a conductive support which is fusedthrough the base and connected to a pin terminal 8. The cathode 6comprises a cylindrical oxidized nickel sleeve 10 upon the exteriorsurface of which is the electron emissive coating 12. Also within theevacuated envelope 2 and mounted on the base 4 are a collector Velectrode 14, a metal shield 16 and a starter electrode 18.-

Electrodes 14 and 18 are connected respectively, through suitableconductive supports to pin terminals 20 and 22. Collector electrode 14is of mesh construction. Starter electrode 18 comprises a wire oftungsten having a fine pointed tip directed toward coating 12 of thecathode, an opening of the collector electrode being aligned with thestarter electrode for unobstructed passage to the coating of primaryelectrons given off by field emission from the starter tip when a highnegative potential with respect to the cathode is impressed upon thestarting electrode.

To prepare the coating 12, and assuming the cathode sleeve to he ofabout outer diameter, a mixture of paste-like consistency is firstobtained by combining Magnesium oxide, reagent grade grams 24 Basicmagnesium carbonate U.S.P. heavy powder (MgCO .Mg(OH ).nH O grams 24Amyl acetate, purified (thinner) cc 300 The paste-like composition isthen milled in a one quart porcelain ball mill jar one-half full ofselected flint pebbles of Vs" to /3" size, the charge just covering thepebbles. The ball mill jar is rotated at a speed of r.p.m. for 20 hoursfollowing which additional amyl acetate, about 420 cc., is added. Themilling reduces the particle size to approximately 0.5 to 2.0 microns.Milling for longer than 20 hours does not appreciably reduce particlesize. Milling for a shorter period of time, for example, 5 hours, givesa wider range of particle size say from 1 to 10 microns. If the coatingmixture is fully thinned prior to milling the relatively uniform andsmall particle size is not obtained.

To apply the composition to the cathode sleeve, the sleeve is rotated atrpm. while sprayed with the milled composition using a spray type gununder an air pressure of 20 lbs. Spraying is continued until the coatingthickness is from 28 to 48 microns and the coating weight from 1.4 to1.6 milligrams per square centimeter.

With a cathode coating of the above described initial compositionprepared and applied as above described, copious and self-sustainedemission of electrons results once the emission is initiated either by astream of primary electrons, as from the point of the starter electrodeof the device, or by a spark coil or as a result of illumination byultraviolet light. When a collector electrode such as electrode 14 ismaintained positive with respect to the cathode by application of asuitable voltage thereto conduction through the tube will be maintained.It is believed that the high rate of electron emission and theself-sustaining feature thereof is due to the porous or spongy characterof the coating when prepared as above described. A radial potentialgradient appears to be set up in the coating which causes avalancheemission of electrons. The addition of basic magnesium carbonate to themagnesium oxide, the line particle size obtained by milling prior tofinal thinning and the high rate of travel of the surface to be coatedtransverse to the direction of the spray are each important to thepreparation of a coating of excellent electron emission characteristics.

The invention has now been described with reference to the preparationof a cold cathode for vacuum tubes. Obviously the new coated electrodeof the invention could be of other than cylindrical shape and could beadvantageously employed in tubes other than the diode illustrated in thedrawing. Various changes in the detailed process described could be madewithout departing from the spirit of the invention or the scope of theaccompanying claims. For example, although amyl acetate has been givenas the thinner, any thinner could be used that does not leave a harmfuldeposit, such as carbon, on the coating and that dries at a suitablerate for the practice of the spray coating process. Various otherproportions of the constituents of the coating could be employed.Although the cathode sleeve rotation during spray coating was given asl50.r.p .m. higher rates couldbe used ifgde sired.

The following is claimed:

1. An electrode for vacuum tubes comprising a metal base having aporous, spongy coatingthereon derived from a paste comprising athinnerand fine particles of magnesium oxide and'basic magnesiumcarbonate, said coating having ahigh electron emission rate.

2. The electrode according to claim 1 wherein said coatingis from 28 to48 microns thick and of a weight per square centimeter of area of 1.4 to1.6 milligrams.

3. The electrode according toclaim 1 wherein said paste comprisessubstantially equal parts by weight of magnesium oxide and basicmagnesium carbonate.

4. The electrode according to claim 1 wherein the particle size ofsaidcoating is'from 1 to 2 microns.

5. The method of preparing a coating having high electron emission ratefor metal electrodes for vacuum tubes comprising preparing a paste ofmagnesium oxide, basic magnesium carbonate and a thinnenmilling thepaste to reduce particle size to about 1 micron, thinning the milledpaste and finally spraying themetal surface a 4 of the electrode withthe thinned paste while rotating the electrode at a rapid rate.

6. The method of preparing a coating having a high electron emissionrate for a metal electrode which comprises preparing a paste ofmagnesium oxide, basic magnesium carbonate and a thinner, milling theprepared paste to reduce particle size to at least about 2 microns,thinning the milled paste and finally spraying the outer surface of themetalelectrode with the thinned milled paste while rapidly rotating theelectrode.

7. The method according to claim 6 wherein the metal electrode isrotated at about 150 revolutionsper minute.

8. The method according to claim 6 wherein the paste is prepared fromsubstantially equal parts by weight of magnesium oxide and basicmagnesium carbonate and wherein the thinner is amyl acetate.

9. The method according to claim 6 wherein milling of the paste iseffected in ball mill jars with flint pebbles of to size at a rate ofrotation of about 90 rpm.

for about 20 hours.

References Cited in the file of this patent UNITED STATES PATENTS1,874,785 Miller et al Aug. 30, 1932 2,433,821 Toorks Dec. 30, 19472,873,218 Dobischek et a1. Feb. 10, 1959

5. THE METHOD OF PREPARING A COATING HAVING HIGH ELECTRON EMISSION RATEFOR METAL ELECTRODES FOR VACUUM TUBES COMPRISING PREPARING A PASTE OFMAGNESIUM OXIDE, BASIC MAGNESIM CARBONTE AND A THINNER, MILLING THEPASTE TO REDUCE PARTICLE SIZE TO ABOUT 1 MICRON, THINNING THE MILLEDPASTE AND FINALLY SPRAYING THE METAL SURFACE OF THE ELECTRODE WITH THETHINNED PASTE WHILE ROTATING THE ELECTRODE AT A RAPID RATE.